The 'Stone of Free Speech' is an object of uncertain origin and purpose. Said to be at least 200 years old, it has been described variously as a historic focal point for political and religious debate, North London equivalent of Speakers Corner or simply a waymarker post, it certainly continues to attract attention.
From the now sadly defunct blog London - One Foot Up, One Foot Down:
I had passed the stone on a previous day, quite unintentionally, so was unable to re-trace my steps. But Roscoe, the walking-talking GPS in this twosome, had his eye on a triangle of grass situated between two lakes.
Discovery! We see the stone monument surrounded by young people and walk up to make sure we have the right site. I want a photo, so ask permission of the group sitting there. "Sure," says one young woman. "Do you know what this is? Some people call it the Stone of Free Speech, but we call it The Great Tampon."